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Ascorbic acid (AA) determination is of high importance in the diagnosis and treatment of diseases. Herein, we report a new ratiometric electrochemical sensor for ascorbic acid using a glassy carbon electrode (GCE) modified with a copper nanoparticle@resin nanosphere nanocomposite (CuNPs@RNS). The ratiometric strategy was established by immobilizing an internal reference (thionine) on the modified electrode using cyclic voltammetry (CV). The performance of the modified electrodes as well as the newly established ratiometric strategy was explored. The sensing platform had good electrocatalytic ability, reproducibility, and stability. In addition, the ratiometric strategy significantly improved the performance of electrochemical sensing with a wide linear range (0.0837 to 15.5 μM) and a low detection limit of 0.0279 μM under the optimal conditions. The original ratiometric electrochemical sensor was successfully applied to monitor ascorbic acid in tablets and urine. These results showed that the sensor provides a new strategy and broad prospects for biomolecular sensing with reliability and high sensitivity. [ABSTRACT FROM AUTHOR]

A biosensor based on the optical signal of gold nanorods (GNRs) and an aptamer as the recognition element is reported to perform the rapid and selective spectrophotometric determination of profenofos. The GNR synthesized by the seed growth method have unique structures which binds with the aptamer through electrostatic adsorption and is well dispersed in a salt-containing solution. Upon the addition of profenofos, specific recognition caused aggregation of the GNRs. The optical signal therefore decreased and the response linearly related to the concentration of profenofos. The results demonstrate that ascorbic acid as the reducing agent plays an important role in the morphology of the GNRs. GNRs with the length-diameter ratio (L/D) of 12 stably combined with the aptamer. For the optimum conditions, spectrophotometric determination of profenofos was achieved using the biosensor with a linear relationship between the absorbance of GNR at 508 nm and the concentration of profenofos from 40 to 460 µg/L. The limit of detection of the method was 31.93 µg/L. In addition, this approach shows good selectivity for the analysis of fruits and vegetable. A valuable new method is reported for the determination of profenofos. [ABSTRACT FROM AUTHOR]

A new electrochemical biosensor was designed for the determination of uric acid by modifying the surface of the glassy carbon electrode with electrodeposited tartrazine and copper nanoparticles (Cu NPs/TAT) nanocomposite. Electrochemical investigations were administered using cyclic voltammetry and differential pulse voltammetry. The modified Cu NPs/TAT nanocomposite electrode was characterized by field emission scanning electron microscopy, mapping, X-ray diffraction, energy-dispersive X-ray spectroscopy, diffuse reflectance spectroscopy, and electrochemical impedance spectroscopy. The results showed an increase in surface conductivity. The modified electrode had linear ranges from 0.03 to 97 µM with a sensitivity of 0.258 µA µM−1 and from 115 to 1100 µM with a sensitivity of 0.0368 µA µM−1. The modified electrode showed a detection limit as low as 2.9 nM at a signal-to-noise ratio of three. In the interference study, this modified electrode had the ability to separate dopamine, uric acid, and ascorbic acid peaks. In addition, the sensor displayed high selectivity and sensitivity, good repeatability, and good stability, and was employed for uric acid determination in human serum samples with satisfactory results. [ABSTRACT FROM AUTHOR]

The determination of antioxidant capacity, total phenolics, and ascorbic acid using accurate, efficient, low cost, and rapid methods has great analytical importance. The antioxidant capacity, total phenolics, and ascorbic acid content of apricots, arugula, banana, cranberries, spinach, and strawberries were investigated with an electrochemical approach and examined for compatibility with conventional methods. The antioxidant activity was determined by ABTS and DPPH assays. The total phenolic content was determined by the Folin Ciocalteu method and ascorbic acid was quantified by high-performance liquid chromatography (HPLC). In order to perform sensitive and simultaneous electrochemical measurements, the surface of a glassy carbon electrode (GCE) was modified by electropolymerization with p-toluene sulfonic acid (TSA). The TSA/GCE modified sensor was used for the first time for the determination of antioxidant capacity and total phenolic content. The surface of the modified sensor was characterized by cyclic voltammetry and scanning electron microscopy. The TSA/GCE was linearly correlated with the differential pulse voltammetry (DPV) for Trolox and gallic acid in 0.1 M NaNO3 and ascorbic acid in pH 7.4 phosphate buffer. Electrochemical methods offer a promising alternative for the determination of antioxidant capacity and total phenolic content due to their simplicity, rapid response, low cost, sensitivity, and reproducibility. [ABSTRACT FROM AUTHOR]

A practical and sensitive electrochemical sensor for hemoglobin (Hb) detection was developed. The main electrochemical reaction was the reduction of Fe (III) in the Hb structure. For the construction of the sensor, Hb was physically immobilized onto a carbon felt electrode (CFE) without using any linker or applying any pre-functioning or electrochemical procedures. After examining the electrochemical mechanism, the experimental conditions, such as pH, were optimized and the analytical characteristics were investigated. A linear range of 0.05 µM to 3.00 µM with a detection limit of 4.4 nM was obtained for Hb. The relative standard deviation value for 3 replicate determinations of 0.05 µM Hb was calculated as1.35%. The developed sensor was successfully applied for Hb detection via standard addition method in synthetic physiological serum sample. Meanwhile, the influences of interfering reagents, such as cysteine, ascorbic acid, bovine serum albumin, and glucose, on Hb detection were also examined. [ABSTRACT FROM AUTHOR]

Alkaline phosphatase (ALP) is a significant biomarker of diseases, such as osteoblastic bone cancer, hepatitis, and myeloma, and hence its determination is essential and important. MnO2/Mn2+ conversion-based magnetic relaxation sensing was developed to directly determine ALP in blood. In the presence of ALP, 2-phospho-L-ascorbic acid trisodium salt was catalyzed to produce ascorbic acid (AA), which reduced MnO2 to Mn2+, causing transverse relaxation time (T2) decay. A linear relationship of ALP from 5 mU·L−1 to 500 mU·L−1 with T2 signal and a low limit of detection (LOD) of 3.32 mU·L−1 were obtained using the optimized conditions of 200 µM of ascorbic acid-phosphate (AAP), 10 µg·mL−1 of MnO2, 37 °C, pH 7.4, and a 30 min reaction time. When directly measuring ALP in serum with the proposed method, the recoveries were from 99.2% to 101.3%, in accordance with values obtained by a commercial assay. In conclusion, a sensitive one-step method for ALP in complex samples was established which has potential for clinical applications. [ABSTRACT FROM AUTHOR]

A surface base polymeric film was fabricated onto a glassy carbon electrode (GCE) via reversible voltammetric scanning in acidic electrolyte containing a (2,4,6-triaminopyrmidine) monomer (TP). The assembled poly(2,4,6-triaminopyrmidine) (PTP) surface material was further modified by electrochemical attachment of tris(2,2'-bipyridine) ruthenium(II) described as Ru(II)-complex. The structure and electrochemical properties of the resulting (Ru-PTP-GCE) surface network were characterized using X-ray photoelectron spectroscopy (XPS), electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and differential pulse voltammetry (DPV). The apparent estimated value of electron transfer rate constant (kapp) of the prepared sensor was three times greater than the virgin PTP. The cooperative effects of the (PTP) and Ru(II)-complex composite increased the efficiency and the stability of the proposed sensor for the selective determination of dopamine (DA) in the presence of ascorbic acid (AA) and uric acid (UA). The detection limit (DL3σ) of DA in this environment was 0.021 μM. The fabricated sensor was successfully applied for DA detection in a blood serum sample with satisfactory recovery. [ABSTRACT FROM AUTHOR]

This work presents a simple, low cost, and portable spectrophotometer based on a digital microscope as a detector for the determination of the antioxidant activity and total phenolic content (TPC). The antioxidant activity was expressed as ascorbic acid equivalents (AAE) using the 2,2-diphenyl-1-picrylhydrazyl radical (DPPH) assay. The TPC value was measured using the Folin-Ciocalteu method. The color of the products was captured using a digital microscope powered through the USB output of a laptop. A digital microscope, held in a 3D printing device was used to take photos of the reaction product that was illuminated with the stable light source of the digital microscope. The pictures were analyzed for quantitation. For the DPPH assay, the developed method provided a working range from 0.6 to 6 mg L−1 of ascorbic acid with a limit of detection (LOD) of 0.2 mg L−1 and a relative standard deviation (RSD) of 2.2%. The TPC method provided a working range from 4.6 to 100 mg L−1 for gallic acid, with a LOD of 1.4 mg L−1 and a RSD of 1.3%. Results of the proposed system agree well with a conventional spectrophotometric method and have been successfully applied to measure the antioxidant activity and TPC of local Thai tea samples. [ABSTRACT FROM AUTHOR]